Acoustic wave information is used by the oil industry to examine and evaluate the earth's subsurface in the exploration and evaluation of valuable subsurface minerals and mineral reservoirs. Acoustic waves are generated and recorded in oil well logging. This is called sonic or acoustic logging. The sonic wave measurement taken in well boreholes is typically the formation compressional slowness (the reciprocal of velocity). However, other acoustic wave types may measured, for example, shear waves and Stoneley waves.
Prior art downhole acoustic wave (seismic) applications have been used to help oil and gas producers better evaluate their reservoirs and maximize hydrocarbon recovery. A wide range of borehole and cross-well geophysical applications provide data to analyze reservoir properties adjacent to the well bore and ahead of the drill bit for both exploration and field development applications.
For example, downhole sonic tools are used for acquiring walkaway or 3-D vertical seismic profiles (VSPs). Down-hole 3-component seismic receivers can be run in combination to simultaneously acquire high-quality 3-component VSP data for detailed structural and stratigraphic reservoir imaging and integration with available surface seismic and well log data.
Conventional borehole seismic services such as velocity surveys and VSPs provide a means to calibrate surface seismic data sets as well as downhole well logs, as well as for images of subsurface geologic structural features. Acquiring Seismic-While-Drilling (SWD) data is a method that uses the drill bit as a downhole energy source to acquire real-time borehole seismic data during the drilling operation without using any special downhole instrumentation. Cross-well seismic imaging techniques are used to evaluate interwell continuity of reservoir units.
3-D Vertical Seismic Profile (3D-VSP) methods may be used to enhance complex reservoir evaluation with conventional 3-D seismic survey data sets. 3D-VSP services provide oil and gas producers with an edge in reservoir characterization. High-resolution 3D-VSP data can be integrated with 3D surface seismic data to provide detailed descriptions of subsurface formation properties and identification of reservoir compartments, not otherwise possible with surface seismic data alone.
Correlating time data and depth data provides detailed structural reservoir models that can help reduce risks and costs in field evaluation and development. Integrating borehole seismic, surface seismic and borehole log data is an important step in constructing an accurate reservoir model. Advanced borehole seismic anisotropy analysis provides for accurate velocity models and improved subsurface imaging. Hydrocarbon reservoirs are buried under an extended overburden consisting of shales and thinly layered sediments, which are often anisotropic. Walkaway VSP surveys are an excellent means to detect subsurface formation anisotropy.
U.S. Pat. No. 6,440,075 to Averkiou discloses an ultrasonic diagnostic imaging system and method for performing nonlinear echo signal imaging with harmonic and intermodulation product (sum or difference frequency) components. Both the harmonic and the intermodulation products are produced by nonlinear effects of tissue or contrast agents and both are advantageously separated from the fundamental transmit components of the echo signals by pulse inversion processing. The use of both types of nonlinear components can improve the signal to noise ratio of the ultrasonic images, and the two types of components can be blended or used in different regions of an image to offset the effects of depth dependent attenuation.
U.S. Pat. No. 3,979,724 to Silverman et al is directed towards a method for determining the position of the drillbit. A Stoneley wave is produced at the surface that propagates through the fluid in the drillstring and produces a seismic signal when it reaches a discontinuity in the fluid path at the drillbit.
Another device used in conjunction with a drillstring is disclosed in U.S. Pat. No. 6,478,107 to Birchak. A compressible fluid is stored in a plurality of chambers within the drill collar. An inlet valve is opened to allow the pressure in a chamber to build up to the pressure of the mud. The inlet valve is then closed and an outlet valve allows the pressurized mud to flow into the annulus between the drill collar and the borehole, thereby generating a pressure wave. This arrangement does not rely on conversion of a Stoneley wave to an acoustic signal.
U.S. Pat. No. 4,993,001 to Winbow discloses a method for the generation of acoustic waves from Stoneley waves for wireline applications. A rotary valve tube wave source is used for producing swept frequency Stoneley waves that are injected into a wellbore. A converter comprising an elongate body that substantially fills the wellbore and has a predetermined shape is positioned at a selected position downhole.
U.S. Pat. No. 6,175,536 to Khan discloses a method for determining a degree of acoustic non-linearity of an earth formation from seismic signals transmitted into the formation from within one wellbore and received from the formation in another wellbore. The seismic signals include two selected discrete frequencies. The method includes spectrally analyzing the received signals, determining from the spectral analysis the presence of a frequency representing a sum of the two selected frequencies, and determining a relative amplitude of the sum frequency with respect to the amplitudes of the two selected discrete frequencies. In a particular embodiment, the method includes determining the presence of a frequency in the spectrally analyzed signals representing the difference between the selected discrete frequencies, and determining the presence of harmonic multiples of one of the two selected discrete frequencies.
There is a need for an efficient method of generating directional sonic wave energy in a wellbore. Low frequency sources can be difficult to direct or target in specific directions relative to a wellbore.